1. Field of the Invention
This invention relates generally to holography, and pertains more particularly to a multi-faceted holographic optical element and method of making and using such an element.
2. Description of the Prior Art
Holographical optical elements must have the correct interference pattern or fringe spacing. In the past such patterns have been recorded by a single exposure on photosensitive film to record the complete pattern, the film then being developed so as to preserve the pattern. As is known, the interference pattern recorded on the film acts like a diffraction grating, or a collection of diffraction gratings, whose orientations and spatial periods are correct for the purpose of diffracting a light wave incident on the film into a desired output pattern, shape or image. The problem has been in constructing holographic optical elements which will have the required set of fringes on the hologram film when the overall interference pattern is complete, and to do so inexpensively.
More recently, computer-generated holograms have been made for the purpose of producing the required set of fringes on the hologram film. This is accomplished by first programming the computer so that it will compute where the fringes ultimately should be. Then artwork is produced from the computed data that contains the desired fringe pattern, this being done with a computer-controlled plotter. Finally, the resulting plot is photoreduced onto a holographic film, the reduction being such that the fringe pattern is sufficiently small to diffract light at whatever appreciable angles are required.
Computer-generated holograms can also be made directly by electron beam writing on a photoresist which is then developed to form the hologram. The advantage of this technique is that the electron beam can write, at the outset, an extremely small pattern. Although no size reduction is needed, the procedure requires very expensive equipment, usually in the million dollar price range. Only a limited number of machines fulfilling this purpose are currently in existence.
Without question, computer-generated holograms are more flexible than other types, but they are often of limited utility because of the high computation costs. Also, whatever plotter inaccuracies exist are carried or continued into the final hologram. The point is that, where the hologram is to contain a relatively large amount of information requiring a large number of fringes to be produced, the involved procedures, even when assisted by a computer, do not provide a satisfactory solution to making an inexpensive hologram. In addition, the maximum possible diffraction efficiency (on the order of only 40.5 percent) is relatively low which is a decided shortcoming. Still further, a computer-controlled hologram produces multiple diffracted orders which limit the working field of a diffractive device such as a scanner. Consequently, while computer-generated holograms do have a high degree of flexibility in producing scan paths, they lack high light efficiency and, when this objection is coupled with the high cost of producing computer-generated holograms, the widespread or general making and use of computer-derived holographic optical elements simply has not as yet evolved.
Where only simple scan patterns are to be produced, it is a relatively easy task to interferometrically construct entire holograms with a single exposure that will produce circular scan patterns and straight line scan patterns. Because the holograms are interferometrically recorded, they can be produced as volume phase holograms which can have 100% diffraction efficiency. These patterns are of such limited utility, however, that even though the holograms are relatively easy to construct, they, for all intents and purposes, have value for only a very restricted class of scanning problems.